<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <link rel="stylesheet" href="css/skin02.css" id="link01">

    <script>
        window.document = function () {
            var oink = document.getElementById('link01');
            var id1 = document.getElementById('btn01');
            var id2 = document.getElementById('btn02');

            id1.onclick = function () {
                oink.href = 'css/skin01.css';
            };

            id2.onclick = function () {
                oink.href = 'css/skin02.css';
            };
        }

    </script>

</head>

<body>
    <h1>网页内容</h1>
    <input type="button" value="皮肤一" id="btn01">
    <input type="button" value="皮肤二" id="btn02">
</body>
</html>